In pelletizing or sintering plants the bulk material to be treated, for example iron ore or zinc ore, is charged onto pallet car which form an endless chain of pallet cars also referred to as traveling grate (cf. FIG. 1). The pallet cars are filled with the bulk material and pass through various treatment stations, in which a thermal treatment of the material, such as drying, firing and cooling, is effected. By a lifting or drive wheel, the pallet car chain is moved through the treatment stations along a top strand, wherein the wheels of the pallet cars are guided in rails. At the end of the top strand, the heat-treated material is dumped by gravity at a lowering wheel or tipping wheel (discharge station), whereupon the empty pallet cars are returned upside down along a bottom strand to the lifting wheel. During conveyance in the top strand and the bottom strand, a contact pressure between the individual pallet cars brings about the necessary chain connection. Gear wheels of the lifting and lowering wheels engage pressure rollers which are arranged on the axle of the pallet cars beside the track rollers, in order to lift the pallet cars and convey the same in vertical direction.
Since the pallet cars with their components are subject to a high thermal and mechanical wear during the production, they must be exchanged after a certain degree of wear or damage. Conventionally, such change requires stopping of the lifting wheel for 5 to 10 minutes, which can lead to an additional thermal damage of the pallet cars in the top strand. Moreover, production must also be stopped in this shut-down period. After the pallet car change, the pelletizing and sintering process must then again adjust to a stable production level. This adjustment phase, which in most cases only is terminated after a few turnaround cycles, likewise has an adverse effect on the product quality.
To avoid the above-described disadvantages of the discontinuous changing devices, it has also been proposed already to perform a continuous pallet car exchange. The pallet car to be changed here is removed from the pallet car chain and a new pallet car is inserted in the free chain position. The change is effected with running traveling grate, so that two different plant positions are required for the partial functions of bringing out and bringing in. The changing device consists of two separate units which operate one after the other.
An example of a continuous changing device is described in U.S. Pat. No. 6,523,673. The changing device is arranged in the lifting wheel bottom strand and in the lifting wheel top strand. Removal of the pallet car is effected in the bottom strand. For this purpose, the guide rail is opened downwards and the pallet car is brought out on a movable conveying means. The conveying means is a sliding table which has the same translatory direction of action as the traveling grate conveyance in the bottom strand. In the top strand, a new pallet car is lifted into the traveling grate by means of a rotatable gripping mechanism, wherein the necessary insertion opening in the curve rails is generated by means of a rotatable flap which is opened upwards. Here as well, however, additional lifting and conveying means are required for removing and inserting the pallet cars.